1. Field of the Invention
The present invention is related to film weave correction systems and more specifically to correcting for horizontal and vertical weave in systems for projecting or transferring film-to-video or video-to-film.
2. Related Art
It is common knowledge that motion pictures are produced by sequentially recording a series of discrete images which are related in time. The initial motion is recorded at a predetermined rate so that the recorded images can be reproduced to give the same motion by projecting or displaying the images at the predetermined rate. Present movie cameras for recording original film are relatively accurate in recording the images at the predetermined rate. For example, 16 mm and 35 mm cameras record images on the film at a rate of 24 frames per second.
Proper positioning of each frame on the film is also important for proper playback of the film. In a movie camera, proper positioning, or registration, of each frame is done by pulling the film down across the lens and aperture and momentarily stopping the film and refixing the frame by fitting one or more pins in the sprocket hole associated with the frame. The frame is then exposed with light through the lens. After the pin is disengaged from the sprocket hole, the film is advanced until the next frame is presented before the lens and aperture. The process is then repeated. The process is repeated at a rate of 24 frames per second, and the frame is registered with respect to the lens and aperture in both vertical and horizontal directions by registration of the pin or pins with the sprocket holes corresponding to each frame. In some systems, sprocket holes on different sides of the film are used for horizontal and vertical registration. Typically, sprocket holes of the Bell and Howell or Cathedral type are used for registration of film. Cameras of current design provide for relatively accurate registration of the film during filming. An optical recorder operates in a similar manner.
Registration through pins may be accomplished either by placing the sprocket holes over one or more pins or by feeding the film into a gate and inserting one or more pins into the sprocket holes of the film. In either case, this process results in recording of the image on the respective frame using the sprocket hole as a reference point. In those projectors where the pin is large enough to contact all four edges of the sprocket hole, the film will be registered in both the vertical and horizontal directions. In those cameras where the registration pin contacts only the upper and lower edges of the sprocket hole, the film frame is registered in a vertical direction by the registration pin and in the horizontal direction by an edge guide against which one edge of the film is pressed.
Replay of the motion picture, for viewing or copying, can take several forms. When the film is in the form of production prints obtained from the original negative, for use in movie theaters, the projector advances the film at the rate of 24 frames per second and registers each frame by combining a sprocket hole with a registration pin or by a combination of sprocket hole registration and edge guiding. In either case, exact registration of the film is limited by the 0.010 mm tolerance in the sprocket hole width and height inherent in each sprocket hole and by the wear that inevitably results by repeated use of pin registration. In a situation where edge guiding is used, the edge guides produce wear on the edges of the film which increases any error being produced during playback of the film. However, each time the film is passed through a sprocket assembly, the edges of the sprocket hole undergo a certain amount of wear, which accumulates with each pass of the film over the sprocket. Therefore, exact registration of the film becomes more and more difficult and the vertical and horizontal weave perceived during playback of the film increases.
During film-to-video transfer or film-to-film transfer, the same problems arise with respect to vertical and horizontal weave. In film-to-film transfer, both the original film and the film to be exposed must be registered to provide suitable image transfer. For film-to-video tape transfer, telecine machines are used to transfer picture information from 35 mm movie film, or other film, to video, formatted for television. One such apparatus is the Rank Cintel telecine apparatus, such as the Mark III C Telecine manufactured by Rank Cintel Ltd. This is a continuous-motion or real-time flying spot scanning telecine apparatus which can transfer film information to video on a real-time basis, e.g., 24 frames per second. This apparatus will be referred to as the Rank Cintel apparatus.
The Rank Cintel apparatus takes 35 mm or 16 mm film and uses a Rank Cintel cathode ray tube (Rank CRT, or Rank scanner) to scan the film, on a frame-by-frame basis, in a noninterlaced fashion. The picture information, i.e. the information recorded on each frame of the film, modulates the light beam from the flying spot scanner and produces voltage variations in a series of photomultiplier tubes. The resulting signal is stored in a digital signal processor (digiscan) and thereafter stored as video signals on an appropriate medium, such as video tape. The video signals are stored on the video tape in an interlaced fashion according to the American standard of 30 frames per second, 2 fields per frame. By way of example, film scanned by the Rank Cintel apparatus at a speed of 24 film frames per second is digitized and stored as a video signal, and is clocked out through a 2-3 sequencing arrangement to be 30 video frames per second as is known in the art. (It will herein be understood that the same comments with respect to use of the Telecine apparatus apply to use for the European standard of 25 frames per second with appropriate changes in scanning speed, video storage format, et cetera.)
With the telecine apparatus, the film is edge guided to minimize horizontal weave. Additionally, vertical movement is monitored by a tachometer sprocket which senses the passage of the first sprocket hole at the beginning of each frame and adjusts the capstan drive to correct for framing error. The sprocket holes used for registering the film in the movie camera are otherwise unused for registration of the film during the film-to-video transfer process. This is especially significant where the film was originally registered using only the sprocket holes as opposed to a combination of the sprocket holes with edge guiding. This minimal film registration produces noticeable vertical and horizontal weave in the video signal when the signal is displayed on a video monitor. As with many other display modes, the edges of video monitor provides a reference point which accentuates any vertical or horizontal weave which may have occurred during the transfer process. The use of mattes or keys also easily points out problems in film positioning. In compositing, or for example where a title is added to picture information, the title is stationary, and any weave or vertical drift in the picture information is readily seen relative to the fixed title.
One solution to the film registration problem is to provide film pulldown as in typical movie cameras and projectors followed by scanning of each frame of the registered film with a flying spot scanner. The Rank Cintel telecine apparatus is modified by removing the Rank film gate and substituting a film gate which provides intermittent film pulldown and registration of the film before the flying spot scanning beam. However, the film-to-video transfer is limited to 40 frames per minute. This works out to less than one frame per second. This intermittent pulldown followed by mechanical registration with the sprocket holes is inherently inaccurate because of movement of the film caused by the registration pins or by wear on the sprocket holes caused by the pins.
Where the film is physically guided through a telecine apparatus on a real-time basis, the film can be edge-guided, in perceptible weave or vertical drift in the resulting transferred picture due both to wear on the edge of the film and to the fact that the original frames were most probably registered through the sprockets rather than the film edge. In the most common arrangement, the film is capstan-driven and is not registered in any way. This results in a more significant amount of horizontal and vertical weave in the film. This is the case even where the capstan-driven Rank apparatus may still have correction for vertical drift of the film because the vertical correction is dependent upon the mechanical tolerances of the capstan and the film.
Holland, U.S. Pat. No. 4,104,680, Goldmark, U.S. Pat. No. 3,234,326, Pickstock, U.S. Pat. No. 3,780,222, and McMann, U.S. Pat. No. 3,856,987, attempted to solve the problem of film registration. Rather than mechanically register each frame with a sprocket hole, these inventors electronically or optically sensed a sprocket hole or synchronization mark on the film for each frame to be scanned. Holland uses the electron beam used to scan the picture information in each frame to sense a sprocket hole. Using the same beam severely limits the amount and integrity of the information produced through the electron beam and the sprocket hole.
Goldmark uses a separate light source to detect pips or optically recorded marks on the film adjacent each scan line. Goldmark suffers the same problems of resolution and integrity of information as does Holland. McMann also uses a separate light source to produce a signal upon the passing of a synchronization mark formed in the film. Goldmark and McMann both address the problem of vertical drift. Pickstock uses the flying spot scanner for the picture information to detect a synchronization mark for correcting weave.
It should be noted that the above-described aspects of film transfer apply to telecine apparatus throughout a range of film rates, such as between 16 and 30 frames per second. They also apply to apparatus for transfer of video to film, use of electron beams for moving picture information transfer, laser transfer, and other apparatus where film registration is desirable.